RESUMEN
Segmental dynamics of relatively short linear polymers are discussed in terms of two distinct contributions, one related to the local segmental motion (alpha relaxation) and the other to polymer-specific effects that reflect Brownian dynamics of the polymer under chain connectivity constraints (Rouse relaxation modes). These two aspects of polymer dynamics are reflected, though differently, in relaxation spectra of different experimental techniques. Two contrasting cases of the (collective) dipolar response (dielectric techniques) versus the individual segmental response (e.g., NMR spin-lattice relaxation spectroscopy) are considered. The second-rank orientational correlation function of an elementary (Kuhn) segment, directly related to NMR observables, is derived in terms of Rouse normal modes. The effect of alpha dynamics is estimated under the assumption of a separation of time scales which, as it is argued, is a necessary precondition of the Rouse approach. The relative magnitude of the polymer-related dynamics is expressed through the number of elementary Rouse units in the chain and the number of Kuhn segments in a Rouse unit. The results are discussed in the context of recent literature.
RESUMEN
We have previously discussed [J. Chem. Phys. 125, 114502 (2006)] that optical Kerr effect (OKE) and depolarized light scattering (DLS) data of molecular liquids reveal, each in their native domain, the same characteristic signatures of the glass transition dynamics; in particular, the intermediate power law of OKE is equivalent with the excess wing of the frequency-domain data, long since known in dielectric spectroscopy. We now extend the discussion to show that the excess wing is an equally common feature in DLS. We further discuss the time-temperature superposition property of OKE data in relation to our DLS and literature dielectric-spectroscopic results, and the merits of their mode coupling theory analyses. Spectroscopic signatures of a liquid-crystal-forming system (nematogen) are discussed in the same frame.
RESUMEN
Recently, heterodyne-detected optical Kerr effect (HD-OKE) spectroscopy was used to study dynamics of supercooled molecular liquids. The studies revealed an apparently new physical phenomenon that had not been reported before from the related depolarized light scattering (DLS), namely, an intermediate power law (nearly logarithmic decay) of the response functions [H. Cang et al., J. Chem. Phys. 118, 2800 (2003)]. Conceptually, HD-OKE and DLS data reflect optical anisotropy fluctuations mainly due to molecular reorientation dynamics in time and frequency domains, respectively. The above-mentioned effects are revealed in the mesoscopic range less, similar1 GHz ( greater, similar100 ps), where no direct comparison of the techniques was reported. In this Communication, we attempt such a comparison of exemplifying HD-OKE literature data of the glass-forming salol (phenyl salicylate), benzophenone, and liquid-crystal forming 4-cyano-4(')-pentylbiphenyl with DLS data of the same systems that we measured down to ca. 200 MHz by a combined tandem Fabry-Perot interferometer plus tandem-grating-monochromator technique. Generally, we find a satisfactory agreement, albeit in some cases with subtle differences at frequencies greater, similar10 GHz. We conclude that, in the mesoscopic dynamic range, HD-OKE and DLS studies provide consistent and comparable information, and therefore their conclusions must agree. We argue that the intermediate power law of HD-OKE is in essence a manifestation of the excess wing of the corresponding frequency-domain data, known long since from broadband dielectric spectroscopy and anticipated from DLS studies of supercooled liquids.
RESUMEN
We discuss the dynamics of the glass-forming liquids glycerol, propylene carbonate, and benzophenone, as revealed in their dielectric behaviour and in depolarized light scattering (DLS) data. Above the melting point, the liquids exhibit two-step stretched relaxation behaviour typical of 'glassy dynamics' at all attainable temperatures [Formula: see text]. There is no sign of a transition to exponential relaxation; rather, the stretching depends only weakly on the temperature. The spectral change, in first approximation, amounts to shifting the α-relaxation peak with the temperature. This behaviour is in contrast with the temperature-dependent spectral shapes observed in the low-temperature state close to T(g). Analysing corresponding dielectric spectroscopic (DS) data for comparisons, similar tendencies are observed, except that the stretching parameters are different. Below a certain temperature in glycerol, the tail of the excess wing, which is clearly pronounced in the DS data, appears also in the DLS spectra, with the same ratio of the wing exponents in DLS versus DS as of the stretching exponents at higher T. In propylene carbonate, the presence of a wing in the DLS data is not immediately obvious. However, its low-temperature DLS spectra are fully compatible with an empirical model for the temperature-dependent spectral shape that adequately describes the DS data of both materials and includes the wing. We were however unable to reconcile the DLS results of glycerol with this model. This, together with the pronounced difference in DLS and DS spectral widths of glycerol, cast doubts over the general validity of universal scaling procedures for different techniques.
RESUMEN
Brillouin-scattering spectra of the molecular glass-forming material propylene carbonate (PC) in the temperature range 140 K to 350 K were analyzed using both the phenomenological Cole-Davidson memory function and a hybrid memory function consisting of the Cole-Davidson function plus a power-law term representing the critical decay part of the fast beta relaxation. The spectra were also analyzed using the extended two-correlator schematic mode-coupling theory (MCT) model recently employed by Götze and Voigtmann to analyze depolarized light backscattering, dielectric, and neutron-scattering spectra of PC [Phys. Rev. E 61, 4133 (2000)]. We assess the ability of the phenomenological and MCT fits, each with three free fitting parameters, to simultaneously describe the spectra and give reasonable values for the alpha-relaxation time tau(alpha).